Electrostatic Discharge (ESD) is often described as the flow of electricity between two electrically charged objects. ESD may occur when differently-charged objects are brought close together and may include the creation of a visible spark. One example of ESD is the discharge of static electricity when a person touches an electrically conductive object.
ESD is a major problem faced by electronic manufacturers that sell semi-conductor devices. Semi-conductor devices are typically sensitive to the presence of static electricity and inadvertent discharge can result in severe damage to the electronics. For example, in looking at the effect of ESD on electronic devices, a human being can be represented as a capacitor (an electrical storage device) of 100 pF and may be charged to a voltage of from 4,000 to 35,000 volts. When the person touches an object, this stored energy may be discharged through the object. While the energy that is transmitted is typically very small, the voltage can still damage sensitive electronics.
It is virtually impossible to protect against any and all levels of ESD, however, two general approaches have been taken to deal with this problem.
A first approach is to increase the length that an arc must travel to reach an underlying Printed Circuit Board (PCB) including sensitive electronic components, such that the voltage required to bridge the air gap must be relatively large. Therefore, the simplest way to provide protection against ESD to some threshold is to keep the PCB as far from a potential discharge source as is possible. This approach is limited in effectiveness due to the structural limitations of where the PCB can be placed relative to the ESD potential.
As second approach has been to provide an insulated barrier that seeks to protect the PCB. For example, if a barrier of solid plastic were placed between the PCB and a potential discharge source, the path for the potential ESD would have significantly more resistance than an air gap.
Both of these solutions seek to prevent any and all ESD from reaching the PCB by increasing the resistance to ground such that it becomes difficult for ESD to occur through the device. However, with the very high voltages that can occur (up to 35,000 volts), this approach has had limited success as the insulting material required to prevent a discharge at such a high potential become unwieldly and very expensive in relation to the relatively low cost of the component that it is supposed to protect. Likewise, the air gap distances required for effective insulation are too large for practical implementation especially in relatively small spaces, the size of the component cannot be increased to a size necessary to prevent all ESD. The result has been that, while some ESD has been prevented, the devices are still subject to damage caused by high ESD voltages. The end result being a damaged or destroyed electronic components.
Other ideas have sought to completely direct the ESD away from the device, in effect, providing a low resistance path that carries the electrical power away from sensitive components. For example, U.S. Pat. No. 8,008,589 (the '589 patent) discloses an electrical switching device including an anti ESD device. The '589 patent teaches “providing on a surface of the electrical switching device an electrically conductive element” where “an electrical charge at a fingertip of the user is, due to the higher electrical conductivity, always transferred to the metallic element and does not penetrate the plastic surface.” Col. 2, II. 1-3 & 8-11. The '589 patent further teaches that “an earth connection which can be designed as desired and via which the electrical charge is carried away in a focused manner” such that “damage to the actual electrical or electronic switching device is effectively avoided, as the charge is not introduced into a delicate circuit of the electrical switching device.” Col. 2, II. 11-18. In particular, the '589 patent teaches that the “metallic element reliably guarantees that no electrical charge penetrates for example a plastic surface of the electrical switching device uncontrolled and damages a circuit.” Col. 2, II. 28-30.
However, a limitation of the '589 patent is that it seeks to again, prevent or stop any ESD from reaching the printed circuit board (PCB) that it is seeking to protect, which is just not possible. The critical feature of the '589 patent is that “the electrical charge is carried away from the ring 4 via the earth connection.” Col. 4, II. 48-49. While the '589 patent may help to reduce the frequency of which ESD reaches the PCB, this configuration can't stop all of it. The electronics on a PCB are electronic in nature and inherently utilize electrical pathways. The very nature of the PCB has a tendency to draw electricity toward it to find a pathway to ground. Likewise, very high voltage can travel along unexpected paths depending on environmental and atmospheric conditions. While it is possible to provide a pathway to direct ESD away from sensitive electronics, it is simply not possible to fully control the path of ESD under many varying conditions. Discharge to the PCB with this system will still result in damage to sensitive electronics.
U.S. Pat. No. 9,072,161 (the '161 patent) is another system that seeks to prevent any ESD from reaching the PCB. As was stated previously, this is virtually impossible to do in a consistent and reliable manner. For example, the '161 patent teaches use of an “Electro-Static shielding layer located on an upper surface of the base layer and can at least covering sensitive areas . . . and an insulating layer for at least covering the Electro-Static shielding layer.” Col. 2, II. 18-22. The '161 patent will suffer from the same problem as the '589 patent in that, it is just not possible to fully prevent any ESD from reaching the PCB even if a shielding is provided over top the of various parts of the PCB and designed to carry charge away from the sensitive electronics.
U.S. Patent Application Publication No. 2016/0242331 (the '331 application) is yet another system that seeks to protect a PCB from ESD. The '331 application takes the same flawed approach as the previous designs, namely, “preventing the infiltration of ESD into the inside of the PCB.” Para. 123.
What is needed then is a system that provides enhanced ESD protection that does not suffer from the problems discussed above.